Hello Andrew,

In R3 2019 SP1 this issue where upon inserting a row the data in cell A1 is removed will be resolved with the introduction of a second parameter to the insertRow method:

skipDataSourceInsert Boolean optional
If passed true, the method does not delete item from the DataSource.

Thus, in databound scenarios when inserting rows outside the databound rows, insertRow()'s parameter skipDataSourceInsert should be set to true to avoid the issue from the KB.
sheet.insertRow(0, true);  -  the second parameter skips the update of the dataSource

We will update the documentation and KB article once R3 2019 SP1 is released.

Regards,
Aleksandar
Progress Telerik

Hello Andrew,

This behavior is expected when setting the dataSource while handling the requestEnd event as this causes an endless loop. You can set the dataSource via the setDataSource() method outside of the event handler, but still add the header rows in the requestEnd event handler.

I modified the example to set the dataSource via the setDataSource() method, to match your scenario. Note that a bug was identified where upon insertion of a row causes the first cell of the column titles to be removed thus the following  was added as a temporary solution:

function onRequestEnd(e) {
      // Check the request type
      if (e.type === 'read') {
        setTimeout(function() {
          var spread = $("#spreadsheet").getKendoSpreadsheet();
          var sheet = spread.activeSheet();
          // Insert the custom header row
          sheet.insertRow(0);
          sheet.insertRow(0);
          sheet.range('A1:C1').merge().value("My Custom Header").textAlign("center").fontSize(50);
          sheet.range('A2:C2').merge().value("some other data").textAlign("center").fontSize(20);
          sheet.range('A1:C2').background('#cccccc')
          sheet.range('A3').value(sheet.dataSourceBinder.columns[0].title)         
        },0);
      }
    }

I have converted this thread to a "Bug Report" in our Public Feedback Portal from where you could track it. Also, the following GitHub issue can also be monitored. 

As a token of gratitude for helping us identify this bug I have updated your Telerik Points accordingly.

Regards,
Aleksandar
Progress Telerik

Hello Andrew,

You can add a custom header by handling the requestEnd event of the dataSource object and manually inserting the header by handling the Spreadsheet client-side API, as described in this knowledge base article. Note however that if you need to have different styling in the header you would need to add a second merged row. In this dojo I have modified the example from the article to meet your requirements.

I hope this solves your issue. Let me know if you have further questions. 

Regards,
Aleksandar
Progress Telerik